docs(runbooks): add "new web app" setup runbook under doc/runbooks/

Document, as a tree-docs tree, the end-to-end procedure to stand up a new
web application on the Arcodange platform — a mechanic spread across the
factory, tools and app repos with non-trivial ordering dependencies.

Covers: Gitea repo creation (org-secret inheritance), Postgres DB + owner
role (factory/postgres/iac), platform Vault declaration (gitea_cicd_<app>
+ policies, tools/hashicorp-vault/iac), the app Helm chart (VSO dynamic
secrets via pgbouncer), the app Terraform (app_roles module), the CI
workflows (tofu apply + image build, incl. the copy-pasted role pitfall),
and ArgoCD registration (factory/argocd/values.yaml). Adds a naming-
conventions concept page and an ordered checklist.

Wires the legacy doc/adr "setup hello world web app" item and the factory
README to the runbook. New docs live under doc/ (singular) per the PR #8
convention.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -80,4 +80,9 @@ classDef done fill:gold,stroke:indigo,stroke-width:4px,color:blue;
 class prepare_hd,nodeId2 done;
 ```
 
+## Documentation
+
+- 📚 [`doc/`](doc/README.md) — ADR (décisions d'architecture) + runbooks.
+- 🚀 [Runbook : mettre en service une nouvelle application web](doc/runbooks/new-web-app/README.md) — dépôt Gitea, base de données, Vault, chart Helm, Terraform, CI, ArgoCD.
+
 🏹💻🪽